1. Field of the Invention
The present invention relates to a plasma deposition method and an apparatus therefor, utilizing electron cyclotron resonance (hereinafter, ECR).
2. Information of the Related Art
In general, a process for manufacturing a semiconductor integrated circuit, a conductor or nonconductor film is deposited on a semiconductor wafer by repeating thin film depositing operation and etching operation so that a desired integrated circuit is formed thereon.
Aluminum wiring is mainly used for a wiring pattern of the integrated circuit, and a SiO.sub.2 or SiOF film is used as the material of a layer insulating film for insulating the wiring. The SiO.sub.2 or SiOF film can be formed with good quality by ECR plasma process which combines microwaves and a magnetic field.
FIG. 6 shows an example of a prior art plasma process apparatus which carries out the ECR plasma process. Microwaves of, for example, 2.45 GHz are introduced into a vacuum container 1 through a waveguide (not shown), and at the same time, a magnetic field with a predetermined intensity, e.g., 875 G, is applied by means of a magnetic coil 10. Plasma generating gases, such as Ar and O.sub.2 gases, are converted into high-density plasmas through the interaction (resonance) between the microwaves and the magnetic field. A reactive gas, such as SiH.sub.4 or SiF.sub.4 gas, is activated to form an ion seed by means of the plasma, and the surface of a semiconductor wafer W on a stage (susceptor) 11 is subjected to sputter etching and deposition at the same time. The sputter etching and depositing operations, which run counter to each other, are controlled so that the deposition is macroscopically superior to the sputter etching.
Generally, in the ECR plasma process, a negative radio-frequency bias voltage of 13.56 MHz from a radio-frequency power supply unit 12 is applied to the stage 11 which bears the wafer thereon, so that as many positive ions as possible are attracted to the wafer to maximize the sputtering effect. In sequential operation, the gases are first fed into the vacuum container 1 at time ta.sub.a, as shown in FIG. 7, and the microwaves and the radio-frequency bias voltage are simultaneously supplied and applied, respectively, at time t.sub.b. The radio-frequency bias voltage is thus applied for the following reason. When the stage is not subjected to any bias, only the deposition is effected in advance, so that there is a high probability that voids will be formed in the aluminum wiring. If the negative bias voltage is applied in the aforesaid manner, on the other hand, the deposition is effected simultaneously with the sputter etching, so that a deposit on the edge portion of the aluminum wiring is influenced mainly by the sputter etching, and film deposition in this region is restrained. As a result, the frontages of recesses between aluminum wires are widened, so that the deposition fully covers a deep part, thus ensuring embedding with fewer voids.
If a radio-frequency bias at a frequency as high as, for example, 13.56 MHz is applied to the wafer W, electrons can move following this frequency. Since ions in the plasma cannot move following the high frequency, however, the electrons accumulate in the surface of the wafer W. When the wafer surface is saturated with the electrons, it acquires a self-bias voltage of a certain level, and a potential distribution is formed in the plane of the wafer W.
This in-plane potential distribution is formed from the following causes. As shown in FIG. 8, magnetic fluxes are not uniformly perpendicular to the wafer surface, and spread out with distance from the central axis of the vacuum container 1. Accordingly, the perpendicularity of the magnetic fluxes is poorer in the peripheral portion of the wafer W than in the central portion, so that the electrons reach the peripheral portion with a delay. Another presumable cause is that the vacuum container 1 on the ground side does not face the wafer W perfectly, as shown in FIG. 6.
If the in-plane potential distribution is not uniform in forming the layer insulating film of a device on, for example, the aluminum wiring on the wafer surface, however, existing insulating films, such as a gate oxide film of a transistor, may be damaged from the following cause, in some cases. Since the substrate of the transistor has the same potential in the plane direction, the gate oxide film is subjected to a potential difference corresponding to the aforesaid potential distribution through the substrate, in the initial phase of film deposition, that is, in the state that the aluminum wiring is practically exposed.
The gate oxide film has a very small thickness of, for example, 50 to 100 angstroms, and its withstand voltage is about 10 volts. In some cases, therefore, the aforesaid potential difference exceeds the withstand voltage, so that the gate oxide film may be deteriorated or broken down in the end.
Thus, in forming, for example, the layer insulating film by the conventional ECR plasma process method, the gate oxide film may possibly be damaged, so that the yield of products cannot be improved.